1. Field of the Invention
The present invention relates to a method for producing an optical fiber preform.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-504080 describes an optical fiber composed of a silica-based glass and having a core doped with an alkali metal. It is stated that the doping of the alkali metal in a core allows the viscosity of the core to be reduced when an optical fiber preform is drawn into an optical fiber, and allows the relaxation of the network structure of the silica glass to proceed; hence, the optical fiber has low attenuation.
As a method for doping a silica-based glass with an alkali metal, a diffusion method is known. The diffusion method is one in which a glass pipe composed of the silica-based glass is heated with an external heat source or a plasma is generated in the glass pipe while the vapor of an alkali metal or alkali metal salt, which serves as a raw material, is fed into the glass pipe, thereby doping the inner surface of the glass pipe with the alkali metal element by diffusion.
After the glass pipe is doped with the alkali metal, the diameter of the resulting glass pipe is reduced. After the reduction in diameter, the inner surface of the glass pipe is etched to remove transition metals, such as Ni and Fe, which are contaminated during the doping of the glass pipe with the alkali metal element. After the etching, the glass pipe is collapsed to form an alkali metal-doped core rod. A cladding part is formed on the outside of the alkali metal-doped core rod to produce an optical fiber preform. The optical fiber preform is drawn to produce an optical fiber. In a glass body doped with an alkali metal element, such as potassium, by a vapor deposition process, some alkali metal element atoms in the glass react with oxygen forming the glass or chlorine into oxide or chloride.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-504080 states that in order to achieve low scattering loss, most desirably, the optical fiber preform is subjected to heat treatment at 1600° C. for 30 hours. Under the conditions, however, voids or crystals originated from the alkali metal element are likely to form. An optical fiber produced by drawing the optical fiber preform has high attenuation.